Dynamically creating keys in a JavaScript associative array

Question

Somehow all examples, while work well, are overcomplicated:

• They use new Array(), which is an overkill (and an overhead) for a simple associative array (AKA dictionary).
• The better ones use new Object(). It works fine, but why all this extra typing?

This question is tagged "beginner", so let's make it simple.

The über-simple way to use a dictionary in JavaScript or "Why doesn't JavaScript have a special dictionary object?":

// Create an empty associative array (in JavaScript it is called ... Object)
var dict = {};   // Huh? {} is a shortcut for "new Object()"

// Add a key named fred with value 42
dict.fred = 42;  // We can do that because "fred" is a constant
                 // and conforms to id rules

// Add a key named 2bob2 with value "twins!"
dict["2bob2"] = "twins!";  // We use the subscript notation because
                           // the key is arbitrary (not id)

// Add an arbitrary dynamic key with a dynamic value
var key = ..., // Insanely complex calculations for the key
    val = ...; // Insanely complex calculations for the value
dict[key] = val;

// Read value of "fred"
val = dict.fred;

// Read value of 2bob2
val = dict["2bob2"];

// Read value of our cool secret key
val = dict[key];

Now let's change values:

// Change the value of fred
dict.fred = "astra";
// The assignment creates and/or replaces key-value pairs

// Change the value of 2bob2
dict["2bob2"] = [1, 2, 3];  // Any legal value can be used

// Change value of our secret key
dict[key] = undefined;
// Contrary to popular beliefs, assigning "undefined" does not remove the key

// Go over all keys and values in our dictionary
for (key in dict) {
  // A for-in loop goes over all properties, including inherited properties
  // Let's use only our own properties
  if (dict.hasOwnProperty(key)) {
    console.log("key = " + key + ", value = " + dict[key]);
  }
}

Deleting values is easy too:

// Let's delete fred
delete dict.fred;
// fred is removed, but the rest is still intact

// Let's delete 2bob2
delete dict["2bob2"];

// Let's delete our secret key
delete dict[key];

// Now dict is empty

// Let's replace it, recreating all original data
dict = {
  fred:    42,
  "2bob2": "twins!"
  // We can't add the original secret key because it was dynamic, but
  // we can only add static keys
  // ...
  // oh well
  temp1:   val
};
// Let's rename temp1 into our secret key:
if (key != "temp1") {
  dict[key] = dict.temp1; // Copy the value
  delete dict.temp1;      // Kill the old key
} else {
  // Do nothing; we are good ;-)
}

Use the first example. If the key doesn't exist it will be added.

var a = new Array();
a['name'] = 'oscar';
alert(a['name']);

Will pop up a message box containing 'oscar'.

Try:

var text = 'name = oscar'
var dict = new Array()
var keyValuePair = text.replace(/ /g,'').split('=');
dict[ keyValuePair[0] ] = keyValuePair[1];
alert( dict[keyValuePair[0]] );

JavaScript does not have associative arrays. It has objects.

The following lines of code all do exactly the same thing - set the 'name' field on an object to 'orion'.

var f = new Object(); f.name = 'orion';
var f = new Object(); f['name'] = 'orion';
var f = new Array(); f.name = 'orion';
var f = new Array(); f['name'] = 'orion';
var f = new XMLHttpRequest(); f['name'] = 'orion';

It looks like you have an associative array because an Array is also an Object - however you're not actually adding things into the array at all; you're setting fields on the object.

Now that that is cleared up, here is a working solution to your example:

var text = '{ name = oscar }'
var dict = new Object();

// Remove {} and spaces
var cleaned = text.replace(/[{} ]/g, '');

// Split into key and value
var kvp = cleaned.split('=');

// Put in the object
dict[ kvp[0] ] = kvp[1];
alert( dict.name ); // Prints oscar.

In response to MK_Dev, one is able to iterate, but not consecutively (for that, obviously an array is needed).

A quick Google search brings up hash tables in JavaScript.

Example code for looping over values in a hash (from the aforementioned link):

var myArray = new Array();
myArray['one'] = 1;
myArray['two'] = 2;
myArray['three'] = 3;

// Show the values stored
for (var i in myArray) {
    alert('key is: ' + i + ', value is: ' + myArray[i]);
}

The original code (I added the line numbers so can refer to them):

1  var text = ' name = oscar '
2  var dict = new Array();
3  var keyValuePair = text.split(' = ');
4  dict[ keyValuePair[0] ] = 'whatever';
5  alert( dict ); // Prints nothing.

Almost there...

• line 1: you should do a trim on text so it is name = oscar.

• line 3: okay as long as you always have spaces around your equal. It might be better to not trim in line 1. Use = and trim each keyValuePair

• add a line after 3 and before 4:

    key = keyValuePair[0];`
  

• line 4: Now becomes:

    dict[key] = keyValuePair[1];
  

• line 5: Change to:

    alert( dict['name'] );  // It will print out 'oscar'
  

I'm trying to say that the dict[keyValuePair[0]] does not work. You need to set a string to keyValuePair[0] and use that as the associative key. That is the only way I got mine to work. After you have set it up, you can either refer to it with numeric index or key in quotes.

All modern browsers support a Map, which is a key/value data structure. There are a couple of reasons that make using a Map better than Object:

• An Object has a prototype, so there are default keys in the map.
• The keys of an Object are strings, where they can be any value for a Map.
• You can get the size of a Map easily while you have to keep track of size for an Object.

Example:

var myMap = new Map();

var keyObj = {},
    keyFunc = function () {},
    keyString = "a string";

myMap.set(keyString, "value associated with 'a string'");
myMap.set(keyObj, "value associated with keyObj");
myMap.set(keyFunc, "value associated with keyFunc");

myMap.size; // 3

myMap.get(keyString);    // "value associated with 'a string'"
myMap.get(keyObj);       // "value associated with keyObj"
myMap.get(keyFunc);      // "value associated with keyFunc"

If you want keys that are not referenced from other objects to be garbage collected, consider using a WeakMap instead of a Map.